Automatic power transmissions having planetary gearsets employ a plurality of torque-transmitting mechanisms. The torque-transmitting mechanisms take the form of stationary type and rotating type. Stationary type torque-transmitting mechanisms are commonly termed brakes, and rotating type torque-transmitting mechanisms are commonly termed clutches.
The stationary type torque-transmitting mechanism has at least one portion of that mechanism; namely, a plurality of disc members, drivingly connected through a spline with a stationary housing. Therefore, a stationary piston can be placed in that housing such that rotating drag forces cannot affect the piston. It is also simpler to direct hydraulic fluid through a stationary housing to a stationary piston.
Rotating type torque-transmitting mechanisms, on the other hand, often employ a rotating clutch piston. The entire input portion of the clutch, namely, piston apply plates and friction discs, is assembled in a rotatable housing. The apply piston cooperates with the housing to form an apply chamber. Hydraulic fluid is supplied to the apply chamber through a rotating joint or connection which requires seal members. The seal members provide a leakage path for a portion of the oil, which is not desirable. Also, with the rotating housing the apply chamber requires a ball dump or centrifugal escape mechanism or a centrifugal dam portion which will counteract the centrifugal forces applied hydraulically to the apply piston when the clutch is disengaged.
More recently, it has been proposed to employ a stationary housing and stationary piston in a rotating type torque-transmitting mechanism. This requires a bearing member to be disposed between the piston and a rotating apply plate. The bearing member absorbs the relative rotation between the apply plate and the apply piston when the clutch is engaged and disengaged. With a stationary piston, the return spring for the apply plate and piston is faced either between the apply plate and a rotating housing of the clutch or between a stationary portion of the housing and the stationary piston. When the return spring is disposed between the stationary piston and the stationary portion of the housing, the apply plate is free and can generate noise within the system. Generally, when the return spring is placed between the rotating housing and the apply plate, a significant bearing load is imposed upon the bearing between the piston and apply plate when the clutch is disengaged as well as when the clutch is engaged. While the bearing load during disengagement is less than the load during clutch engagement, this load is still significant and contributes to the drag losses of the transmission, thereby reducing the overall economy of the vehicle drivetrain.